/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;
import java.io.Serializable;

/** An ordering that uses the natural order of the values. */
@GwtCompatible(serializable = true)
@SuppressWarnings("unchecked") // TODO(kevinb): the right way to explain this??
final class NaturalOrdering extends Ordering<Comparable> implements Serializable {
    static final NaturalOrdering INSTANCE = new NaturalOrdering();

    private transient Ordering<Comparable> nullsFirst;
    private transient Ordering<Comparable> nullsLast;

    @Override
    public int compare(Comparable left, Comparable right) {
        checkNotNull(left); // for GWT
        checkNotNull(right);
        return left.compareTo(right);
    }

    @Override
    public <S extends Comparable> Ordering<S> nullsFirst() {
        Ordering<Comparable> result = nullsFirst;
        if (result == null) {
            result = nullsFirst = super.nullsFirst();
        }
        return (Ordering<S>) result;
    }

    @Override
    public <S extends Comparable> Ordering<S> nullsLast() {
        Ordering<Comparable> result = nullsLast;
        if (result == null) {
            result = nullsLast = super.nullsLast();
        }
        return (Ordering<S>) result;
    }

    @Override
    public <S extends Comparable> Ordering<S> reverse() {
        return (Ordering<S>) ReverseNaturalOrdering.INSTANCE;
    }

    // preserving singleton-ness gives equals()/hashCode() for free
    private Object readResolve() {
        return INSTANCE;
    }

    @Override
    public String toString() {
        return "Ordering.natural()";
    }

    private NaturalOrdering() {}

    private static final long serialVersionUID = 0;
}
